Method of manufacturing semiconductor devices, corresponding apparatus and semiconductor device

ABSTRACT

A leadframe has a die pad area and an outer layer of a first metal having a first oxidation potential. The leadframe is placed in contact with a solution containing a second metal having a second oxidation potential, the second oxidation potential being more negative than the first oxidation potential. Radiation energy is then applied to the die pad area of the leadframe contacted with the solution to cause a local increase in temperature of the leadframe. As a result of the temperature increase, a layer of said second metal is selectively provided at the die pad area of the leadframe by a galvanic displacement reaction. An oxidation of the outer layer of the leadframe is then performed to provide an enhancing layer which counters device package delamination.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.17/108,187, filed Dec. 1, 2020, now U.S. Pat. No. 11,610,849, whichclaims the priority benefit of Italian Application for Patent No.102019000022641, filed on Dec. 2, 2019, the contents of which are herebyincorporated by reference in their entireties to the maximum extentallowable by law.

TECHNICAL FIELD

The description relates to manufacturing semiconductor devices.

One or more embodiments may be applied to manufacturing integratedcircuits (ICs).

BACKGROUND

Providing packaged semiconductor devices with improved resistance topackage delamination represents a growing trend in manufacturingsemiconductor devices (e.g., for the automotive sector).

High resistance to package delamination may be achieved with fabricationprocesses in which the top layer of the leadframe (e.g., a silver layer)is provided (e.g., coated) with a so-called enhancing layer of amaterial having higher affinity with the package molding compound (e.g.,epoxy molding compounds).

It is noted that the presence of an enhancing layer covering theleadframe may negatively affect the wettability of the leadframesurface, possibly having an adverse impact on the soft solder die attachprocess, i.e., the process of attaching a semiconductor die on the diepad area of the leadframe via soft-solder attaching material.

Despite the extensive activity in the area, further improved solutionsare desirable.

There is a need in the art to contribute in providing such improvedsolutions.

SUMMARY

One or more embodiments may relate to a method.

One or more embodiments may relate to a corresponding apparatus.

One or more embodiments may relate to a corresponding semiconductordevice (e.g., an integrated circuit).

One or more embodiments may involve (selective) deposition of a noblemetal layer (e.g., gold) on the die pad area of the leadframe viagalvanic displacement reaction enhanced by light (e.g., laser)radiation, prior to forming the enhancing layer via an oxidationreaction.

One or more embodiments may rely on the recognition that a layer of anoble metal (e.g., gold) at the die pad area will not get oxidized (orwill get only slightly oxidized) during formation of the enhancinglayer, thereby preserving the wettability of the die pad area whichfacilitates soft-solder die attachment.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example only,with reference to the annexed figures, wherein:

FIG. 1 is an exemplary representation of a semiconductor device adaptedto be manufactured according to embodiments, and

FIG. 2 is exemplary of a possible manufacturing step.

DETAILED DESCRIPTION

In the ensuing description, one or more specific details areillustrated, aimed at providing an in-depth understanding of examples ofembodiments of this description. The embodiments may be obtained withoutone or more of the specific details, or with other methods, components,materials, etc. In other cases, known structures, materials, oroperations are not illustrated or described in detail so that certainaspects of embodiments will not be obscured.

Reference to “an embodiment” or “one embodiment” in the framework of thepresent description is intended to indicate that a particularconfiguration, structure, or characteristic described in relation to theembodiment is comprised in at least one embodiment. Hence, phrases suchas “in an embodiment” or “in one embodiment” that may be present in oneor more points of the present description do not necessarily refer toone and the same embodiment. Moreover, particular conformations,structures, or characteristics may be combined in any adequate way inone or more embodiments.

Throughout the figures annexed herein, like parts or elements areindicated with like references/numerals and a corresponding descriptionwill not be repeated for brevity.

The references used herein are provided merely for convenience and hencedo not define the extent of protection or the scope of the embodiments.

FIG. 1 is a schematic representation of a semiconductor device 10 suchas an integrated circuit (IC) observed in a plan (top) view.

As currently known in the art, together with other elements/features notvisible in the figure, a device 10 as exemplified herein may comprise aso-called leadframe 12 having a (central, for instance) die pad area 14and (at least) one semiconductor chip or die 16 attached onto the diepad area 14 of the leadframe 12 via a soft-solder process.

The designation “leadframe” (or “lead frame”) is currently used (see,for instance, the Consolidated Glossary of USPC Terms of the UnitedStates Patent and Trademark Office) to indicate a metal frame whichprovides support for an integrated circuit chip or die as well aselectrical leads to interconnect the integrated circuit in the die orchip to other electrical components or contacts.

Essentially, a lead frame comprises an array of electrically-conductiveformations (leads) which, from an outline location, extend inwardly inthe direction of a semiconductor chip or die thus forming an array ofelectrically-conductive formations from a die pad configured to have atleast one semiconductor chip or die attached thereon.

A package 18 can be molded onto the semiconductor die or dice 16attached onto the die pad area 14 of the leadframe 12 to provide adevice package having the external (distal) tips of the leads in theleadframe 12 protruding from the package 18.

The general structure and manufacturing process of a semiconductordevice 10 as exemplified in FIG. 1 (including the provision of variousadditional elements such as wire bonding coupling the leads of theleadframe to the semiconductor die or dice, and so on, not visible inthe figure) are well known to those of skill in the art, which makes itunnecessary to provide a more detailed description herein.

A conventional solution for manufacturing a device such as thesemiconductor device 10 exemplified herein may involve providing aleadframe 12 in the form of a (ribbon-like) strip of metal material suchas copper. Such a strip may comprise plural sections 12. Each of theseincludes a respective die pad area 14 onto which respectivesemiconductor dice can be attached.

The various sections of the ribbon-like structure can be eventuallyseparated (“singulated”) prior to or following molding of respectivepackages 18 to provide individual devices.

A conventional solution in implementing a process as discussedpreviously involves forming onto the metal material (e.g., copper) ofthe leadframe 12 a so-called enhancing layer having higher affinity withthe molding compound 18 which is eventually molded onto the leadframe 12and the semiconductor die or dice 16 attached thereon.

Such a molding compound may conventionally comprise resin material suchas epoxy resin molding compound (EMC, Epoxy Molding Compound).

Providing the enhancing layer may involve providing onto the basicmaterial of the leadframe 12 (a metal such as copper, for instance) acoating of another material (a metal such as silver, for instance) whichis processed to form the enhancing layer. For instance, according to thetreatment process designated NEAP 4.0 (NEAP=Non-Etching AdhesionPromoter), an upper layer (10-30 Å of thickness, 1 Å=10⁻¹⁰ m) of silveroxide (AgO_(x)) is formed “on top” of the silver coating by oxidizingthe silver layer.

As discussed previously, while promoting good adhesion with the packagecompound, the enhancing layer was found to adversely affect theattachment process of the semiconductor die or dice 16 onto the die padarea 14 of the leadframe 12.

Even without wishing to be bound to any specific theory in that respect,the enhancing layer (silver oxide) may negatively affect “wettability”of the leadframe material (copper coated by silver) by soft-solderattach material.

A composition of Pb 95%/Sn 5% or sometimes 1-2% Ag and Sn balance may beexemplary of such a soft-solder attach material.

In order to preserve satisfactory wettability of the die pad area 14,one or more embodiments may involve selectively depositing a layer of anoble metal (e.g., a metal selected out of palladium (Pd), platinum(Pt), and gold (Au), preferably gold) on top of the silver coating ofthe leadframe 12 at the die pad area 14, prior to performing oxidationof the silver layer to form the enhancing layer (which may comprise,e.g., hydroxylated silver oxide).

Selective deposition of a metal layer on certain areas mayconventionally be achieved by using a mask (mechanical or photoresist).Such conventional approach has the drawbacks of low throughput, highcost of the materials (e.g., of the photoresist) and need for dedicatedtools.

In order to mitigate such drawbacks, in one or more embodiments thesilver-coated leadframe may be coated (e.g., plated) with a noble metalsuch as gold at selected areas by resorting to a galvanic displacementreaction. In particular, it is noted that dipping a piece of silver (ora silver-coated object, such as the leadframe 12) into a solutioncontaining Au⁺ ions, the gold tends to coat the surface of silver, andsome silver gets dissolved into the solution in the form of Ag⁺ ions(i.e., a metal displacement reaction takes place). The Nernst formulaE=E⁰+nRT·ln(Ox/Red) or E=E⁰0.059·Log(Ox/Red) describes thethermodynamics of the redox reaction involved in galvanic displacement.It is noted that temperature plays an important role in determining thedynamics of the galvanic displacement reaction.

As exemplified in FIG. 2 , in one or more embodiments the silver-coatedleadframe 12 to be selectively coated (e.g., plated) with gold may bedipped into an aqueous solution S (e.g., contained in a container orvessel C) of gold complex (e.g., having high free cyanide content) at alow temperature (e.g., about 3° C. to 8° C., preferably about 5° C.). Atlow temperature, the kinetics of the galvanic displacement reaction(gold reduction vs. silver oxidation) is extremely slow, therebyresulting in almost negligible gold deposition.

One or more embodiments may contemplate scanning a laser beam LB ofsuitable wavelength (e.g., 532 nm) and pulse duration (e.g., in themicrosecond range) through the aqueous solution S on the area to beplated, i.e., the die pad area 14 of the leadframe 12. For instance, thelaser beam LB may be produced by a laser source LS and directed towardsthe selected area 14 by means of a mirror M.

In one or more embodiments, pulse duration and/or power of the laserbeam LB and/or overall exposure time may be tuned so to result in theradiation energy being applied on the die pad area 14 of the leadframe12 at an amount of about 5 mJ/mm² to 500 mJ/mm², preferably about 100mJ/mm².

Scanning the laser beam LB on selected areas of the leadframe 12 maythus result in local increase of the temperature of the base material(i.e., the metallic leadframe) and the neighboring volume V of aqueoussolution, which facilitates achieving the electrochemical conditionswhich lead to the (vigorous) deposition of a more noble metal (e.g.,gold) by galvanic displacement reaction with a less noble metal (e.g.,silver). For instance, in one or more embodiments the temperature of theleadframe 12 may be increased locally (e.g., only at the die pad area14) to a temperature between about 15° C. to 65° C., preferably about50° C.

In one or more embodiments, the laser wavelength may be selected toavoid high absorbance by the aqueous solution. For instance, in one ormore embodiments the radiation may be absorbed by the solution S lessthan about 30%, or even less. This may facilitate increasing thetemperature of the base metal (i.e., of the leadframe 12) andtransferring heat to the solution, and not vice versa.

In the exemplary case of an aqueous solution containing gold particles(ions), the wavelengths falling within the spectrum of typical gold saltsolutions (i.e., yellow) should be avoided. Additionally, watermolecules may provide absorption in the infrared range, which shouldtherefore also be avoided. Thus, in one or more exemplary embodiments,“green” radiation having a wavelength of 495 nm to 570 nm, preferably520 nm to 540 nm, more preferably at 532 nm, may be selected for thelaser beam LB. Such a green radiation may be obtained by a Nd-Yag solidstate laser doubled in frequency.

In one or more embodiments, a (very thin) gold layer may be plated alsoon unwanted areas of the leadframe 12 (e.g., outside of the die pad area14) due to residual displacement reaction on areas which are notilluminated by the laser radiation. Mild back stripping may be performedon such areas to remove the (thin) undesired gold layer.

In one or more embodiments, the laser source LS may be programmed (e.g.,via software) to scan the laser beam (only) on certain selected areas ofthe leadframe 12 (e.g., the die pad area 14).

One or more embodiments may thus be advantageous in reducingmanufacturing costs of integrated circuits (e.g., because a masking stepis not necessary).

One or more embodiments may be applied on three-dimensional parts of theleadframe 12, where masks cannot be implemented.

In one or more embodiments, after coating the die pad area 14 with anoble metal by galvanic displacement reaction enhanced by laser, theleadframe 12 may be subject to oxidation processing to provide a silveroxide enhancing layer on the remaining parts of the leadframe 12,thereby preserving wettability of the die pad area 14 towardssoft-solder attach material.

As exemplified herein, a method of manufacturing semiconductor devices(e.g., 10) may comprise:

-   -   providing a leadframe (e.g., 12) having a die pad area (e.g.,        14), the leadframe comprising an outer layer of a first metal        having a first oxidation potential,    -   contacting said leadframe with a solution (e.g., an aqueous        solution S) containing a second metal having a second oxidation        potential, the second oxidation potential being more negative        than the first oxidation potential,    -   applying radiation energy (e.g., a light beam or a laser beam        LB) to the die pad area of the leadframe contacted with said        solution to locally increase the temperature of the leadframe,        wherein a layer of said second metal is selectively provided at        the die pad area of the leadframe by galvanic displacement        reaction,    -   providing an enhancing layer onto said leadframe by oxidation of        said outer layer of the leadframe, the enhancing layer        countering device package delamination,    -   attaching onto said die pad area at least one semiconductor die        (e.g., 16) via soft-solder die attach material, and    -   forming a device package (e.g., 18) by molding package material        onto the at least one semiconductor die attached onto said die        pad area of the leadframe.

As exemplified herein, the solution may have a temperature of about 3°C. to 8° C., preferably about 5° C.

As exemplified herein, the solution may comprise metal complexes and/ormetal ions of said second metal.

As exemplified herein, the radiation energy may comprise laser radiationat a wavelength which is absorbed by the solution less than about 30%.

As exemplified herein, the first metal of said outer layer of theleadframe may comprise silver, and said solution may contain at leastone second metal selected out of the group comprising palladium,platinum and gold.

As exemplified herein, the second metal may comprise gold and theradiation energy may comprise laser radiation at a wavelength of about495 nm to 570 nm, preferably about 520 nm to 540 nm, more preferablyabout 532 nm.

As exemplified herein, the temperature of the leadframe may be locallyincreased to about 15° C. to 65° C., preferably about 50° C., byapplying said radiation energy on the die pad area.

As exemplified herein, the radiation energy may be applied on the diepad area of the leadframe at an amount of about 5 mJ/mm² to 500 mJ/mm²,preferably about 100 mJ/mm².

As exemplified herein, back stripping may be performed to remove saidlayer of said second metal from areas of the leadframe other than thedie pad area.

As exemplified herein, an apparatus may comprise:

-   -   a vessel (e.g., C) configured to maintain a leadframe in contact        with a solution containing said second metal, and    -   a radiation energy source (e.g., LS) configured to scan a        radiation energy beam onto the die pad area of said leadframe in        said vessel to locally increase the temperature of the        leadframe, wherein a layer of said second metal is selectively        provided at the die pad area of the leadframe by galvanic        displacement reaction.

As exemplified herein, a semiconductor device may comprise:

-   -   a leadframe having a die pad area,    -   at least one semiconductor die,    -   soft-solder die attach material attaching said at least one        semiconductor die onto said die pad area,    -   a device package of package material molded onto the at least        one semiconductor die attached onto said die pad area of the        leadframe,

wherein:

-   -   the leadframe comprises an outer layer of a first metal having a        first oxidation potential,    -   the die pad area of the leadframe is provided with a layer of a        second metal resulting from a galvanic displacement reaction,        the second metal having a second oxidation potential more        negative than the first oxidation potential,    -   said outer layer of the leadframe is oxidized to provide an        enhancing layer onto said leadframe, the enhancing layer        countering device package delamination, wherein said die pad        area of the leadframe is at least partially exempt from said        enhancing layer, and    -   soft-solder die attach material attaches the at least one        semiconductor die onto said die pad area, the soft-solder die        attach material provided where the die pad area of the leadframe        is exempt from said enhancing layer.

Without prejudice to the underlying principles, the details andembodiments may vary, even significantly, with respect to what has beendescribed by way of example only, without departing from the extent ofprotection.

The claims are an integral part of the technical teaching providedherein in respect of the embodiments.

The extent of protection is defined by the annexed claims.

1. An apparatus for selectively providing a layer of a second metal at adie pad area of a leadframe including an outer layer of a first metal,wherein an oxidation potential of the second metal is more negative thanan oxidation potential of the first metal, the apparatus comprising: avessel configured to maintain said leadframe in contact with a solutioncontaining said second metal; and a radiation energy source configuredto scan a radiation energy beam onto the die pad area of said leadframein said vessel, said radiation energy beam causing a local increase atemperature of the leadframe which results in a selective provision of alayer of said second metal at the die pad area by a galvanicdisplacement reaction.
 2. The apparatus of claim 1, wherein said secondmetal comprises gold and wherein said radiation energy comprises laserradiation at a wavelength of 495 nm to 570 nm.
 3. The apparatus of claim2, wherein the wavelength is between 520 nm and 540 nm.
 4. The apparatusof claim 3, wherein the wavelength is 532 nm.
 5. The apparatus of claim1, wherein said solution has a temperature of 3° C. to 8° C.
 6. Theapparatus of claim 1, wherein said solution comprises at least one ofmetal complexes of said second metal or metal ions of said second metal.7. The apparatus of claim 1, wherein said first metal comprises silver,and said solution contains at least one second metal selected from agroup consisting of: palladium, platinum and gold.
 8. The apparatus ofclaim 1, wherein the local increase in temperature by applying saidradiation energy on the die pad area is to a temperature level of 15° C.to 65° C.
 9. The apparatus of claim 8, wherein the temperature level is50° C.
 10. The apparatus of claim 1, wherein the radiation energy isapplied on the die pad area of the leadframe at an amount of 5 mJ/mm² to500 mJ/mm².
 11. An apparatus, comprising: a vessel containing a solutionincluding a second metal; a radiation energy source configured togenerate radiation energy beam; a scanning device configured to directthe radiation energy beam into the solution; wherein said vessel isconfigured to receive a leadframe including a die pad area having anouter layer of a first metal; wherein an oxidation potential of thesecond metal is more negative than an oxidation potential of the firstmetal; and wherein said scanning device is controlled to scan theradiation energy beam onto the die pad area of said leadframe in saidvessel to form a layer of said second metal at the die pad area by agalvanic displacement reaction.
 12. The apparatus of claim 11, whereinapplication of said radiation energy beam onto the die pad area of saidleadframe causes a local increase a temperature of the leadframe. 13.The apparatus of claim 11, wherein said vessel is configured to maintainsaid leadframe in contact with the solution.
 14. The apparatus of claim11, wherein said solution comprises at least one of metal complexes ofsaid second metal.
 15. The apparatus of claim 11, wherein said solutioncomprises metal ions of said second metal.
 16. The apparatus of claim11, wherein said first metal comprises silver, and said solutioncontains at least one second metal selected from a group consisting of:palladium, platinum and gold.
 17. The apparatus of claim 11, wherein theradiation energy source generates the radiation energy beam with aradiation energy at an amount of 5 mJ/mm² to 500 mJ/mm².
 18. Theapparatus of claim 11, wherein said radiation energy comprises laserradiation at a wavelength between 495 nm and 570 nm.
 19. The apparatusof claim 11, wherein said radiation energy comprises laser radiation ata wavelength between 520 nm and 540 nm.
 20. The apparatus of claim 11,wherein said radiation energy comprises laser radiation at a wavelengthwhich is absorbed by the solution less than about 30%.